Miscellany

The importance of W. H. Bragg

As a 'scientific grandson' of W.H. Bragg (via J.J. Robertson), I have been disappointed that the senior Bragg's role in the development of X-ray crystallography has tended to be played down. The development of the subject depended on the experience W.H. Bragg gained as Professor of Mathematics and Experimental Physics at Adelaide University in South Australia. Bragg was appointed to the chair in 1885 to build a department. At the age of 42 he began research on α-particles. He acquired experience in the procedures for determining the intensities of beams of such particles using ionization chambers. He was elected FRS in 1907, and in 1909, he accepted appointment to the chair of physics at Leeds University.1

During the summer of 1912, father and son had discussions on the recent observations of Friedrich and Knipping and on von Laue’s interpretation. When W.L. Bragg, returned to Cambridge, he deduced the relationship, nl = 2dsin θ. It became evident that the relationship was 3 dimensional and was associated with 3 parameters necessary to adjust a specific plane to the correct orientation for an X-ray reflection to occur. The problem became (a) how to orient a crystal so that a given hkl plane was in the appropriate orientation and (b) how to measure the intensity of the reflected X-ray beam. Step (a) was well known to mineralogical crystallographers familiar with the optical goniometer. Step (b) had, till then, been based on the use of photographic plates. At this juncture, the critical role of W.H. Bragg and his familiarity with ionization chambers is evident.2

With the expertise of Father Bragg, the ionization spectrometer (essentially the later X-ray diffractometer) was created. With the excellent energy and intensity resolution of the new instrument, father and son were able to study scan procedures, the relevance of integrated intensity, etc., the prelude to structure determination of simple crystals.3

Without Father Bragg's skills, the early development of X-ray crystallography could have been much slower, and, indeed, might have occurred elsewhere.

[1] Jenkin, J.G. (1986). The Bragg Family in Adelaide: A Pictorial Celebration. The University of Adelaide Foundation in conjunction with La Trobe University.
[2] Caroe, GM. (1978). William Henry Bragg 1862-1942 Man and Scientist. Cambridge University Press, pp. 74-75
[3] Bragg, W.H. (1914). The Intensity of Reflexion of X-rays by Crystals. Philos. Mag. 27, 881-899. [Reproduced in Acta Cryst. (1969), A25, 3-11].

A. McL. Mathieson